1. Field of the Invention
This invention relates generally to a speaker cabinet and is directed more particularly to a speaker cabinet having an improved baffle plate upon which one or more speakers are mounted.
2. Description of the Prior Art
In the prior art, speaker cabinets have assumed many different shapes. One type of speaker cabinet has been of the box type, such as an air-tight base reflex type or the like, which has provision for sound absorption in the speaker cabinet or attached to the inner wall of the speaker cabinet. A network circuit is assembled on the speaker cabinet and a speaker unit is attached to an aperture in the baffle plate at either its front or rear side. In this case, the baffle plate is generally formed so that its surface is flat. Further, a grille net is attached to the outside of the baffle plate in such a way that the grille net faces the surface of the baffle plate at such a predetermined distance as to protect the speaker vibration element and which also serves as an ornament.
In some prior art speaker cabinets, a grille net is attached directly to the entire surface of a baffle plate to cover the vibration element. With such speaker cabinets there is the disadvantage that the grille net cannot be detached from the baffle plate, and the sound quality is deteriorated because the grille net covers the surface of the vibration plate.
As described above, since the baffle plate of the prior art speaker cabinet is formed to have a surface which is flat, the sound waves emitted from the speaker vibration element which is an actual sound source partially propagates along the surface of the baffle plate and then arrives at its edge where the portions of the baffle plate meet with the side, top and bottom walls of the cabinet. Due to the abrupt change in sound impedance which occurs at the edge, a sound wave is difracted at the edges. This gives the effect that the edge is serving as an imaginary sound source which emits therefrom a low intensity second sound wave. As a result, interference occurs between the sound wave from the speaker vibration element and the second wave from the imaginary sound source at the edges of the baffle plate. The frequency of the imaginary sound souce is dependent on a factor determined by the distance between the speaker vibration element and the end of the baffle plate. As a result, the sound pressure level at a point spaced from the speaker by a predetermined distance is altered in the vicinity of some specific frequency. In particular, a trough is produced in the sound pressure level at some particular frequency in the frequency response characteristic as measured in its middle band. This produces a deterioration of the sound quality. This deterioration becomes worse as the number of edges increases which exist at the same distance from the speaker unit.
With such a prior art speaker there exists a sound wave emitting not only from the vibration element of the speaker unit but also a sound wave from the baffle plate which is vibrated slightly by a sound wave produced in the cabinet. In the prior art where the baffle plate is a flat plate its vibration mode becomes specific thereto, and is limited to a specific frequency, with the result that the sound quality is seriously affected. This deterioration also seriously affects the directivity of the speaker.